The increase in anthropogenic emissions has been accompanied in recent years with an increase in research efforts to curb them. Along with criteria pollutants such as oxides of sulphur (SOx) and oxides of nitrogen (NOx), a lot of emphasis has also been placed on reducing the CO2 emission from stationary sources of these pollutants, such as coal-fired power plants.
Traditional techniques for NOx control include: combustion furnace modifications such as low NOx burners; flue gas recirculation; reburning of the fuel such as coal; staged combustion; post-combustion NOx reduction such as selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR); and other similar technologies. The post-combustion NOx reduction technologies convert NO to nitrogen (N2) in a reducing atmosphere in the presence or absence of a catalyst.
Carbonaceous material has been studied extensively for removal of NO from combustion exhaust gases. The carbonaceous material such as coal or char provides a reducing atmosphere for the NOx gas, by getting oxidized to CO or CO2, and reducing NOx to N2. The presence of oxygen (O2) in the gaseous mixture enhances the NO reduction. The C-oxygen reaction is favored over C—NO reaction; however, the C-oxygen reaction results in the formation C(O) and C(O2) complexes, which when desorbed from the surface, result in active sites on the carbon surface. These active sites are then used for the C—NO dissociative chemisorption, which subsequently leads to the escape of the adsorbed N as gaseous N2. Therefore, the O2 present in coal combustion flue gas aids the reduction of NOx.
Further, carbonaceous material impregnated with alkali and alkaline earth metals (such as Na, K, Ca) and some transition metals (Cu, Ni, Co, Fe) is known to catalyze NO reduction by carbonaceous material. The mineral matter present inherently in coal char also catalyzes the NO reduction. The metal oxides provide binding sites for the oxygen which facilitates the reaction between C and O. Thus, there exists a multitude of research on several parameters—the different types of metal oxides present in the carbonaceous material such as char, amount of their loading, effect of the various other gaseous species involved such as SOx, O2, CO2, etc. on the overall NO reduction.
NOx reduction by using carbonaceous materials derived from different coal types (bituminous, lignite, etc.) has been extensively studied at The Ohio State University (“OSU”). The OSU-patented CARBONOX process was developed as a result of these studies, and was successfully demonstrated at the pilot scale.
OSU's recent research efforts have also led to the development of the Carbonation-Calcination Reaction (CCR) Process for removal of CO2 and SO2 from coal-combustion flue gas. The CCR Process makes use of a calcium-based sorbent to simultaneously capture CO2 and SO2. In this process, calcium oxide (CaO) reacts with CO2 to form CaCO3. The CaCO3 is then decomposed in another reactor to release high-purity CO2 for sequestration and regenerate the CaO. CaO also reacts with SO2 in the presence of O2 to form CaSO4. Since CaSO4 does not decompose at the CCR operating conditions, a purge stream of solids is maintained to avoid CaSO4 build-up in the solids loop. The CCR Process has also been demonstrated at 120 kWth subpilot scale.
The novel invention described herein was successful in combining the two technologies—CARBONOX and CCR—to form a novel process for the simultaneous removal of CO2, SOx and NOx from a gas mixture in general and coal-combustion flue gas in particular. In this novel process, a calcium sorbent and a carbonaceous material (like char, etc) will be contacted with the flue gas in a single reactor at an appropriate temperature and hence, simultaneous removal of CO2, SOx and NOx will be achieved in a single step. NOx reduction can be ensured by the addition of excess carbonaceous material, and the unreacted carbonaceous material will be used as a fuel in the second reactor to drive the endothermic regeneration reaction of the calcium sorbent.
Exemplary embodiments according to the inventive concept are an advancement over the prior art. As stated herein, embodiments of the inventive concept combines the removal of CO2, SOx and NOx into a single step. In the exemplary process of the inventive concept, a carbonaceous material is introduced into a carbonator. A sorbent, metal oxide preferably CaO, is also introduced into the carbonator. The product of the carbonator is then fed to a particle collection device wherein the clean flue gas is separated from the solids (CaCO3, CaSO4, unreacted char, and unreacted sorbent (CaO)). The solids are then directed to a calciner. The unreacted char will be combusted in oxy-combustion mode to supply heat for the endothermic sorbent regeneration reaction.
After the calciner, another PCD is provided that is used to separate the high-purity CO2 stream from the solids exiting the calciner. The regenerated sorbent exiting the calciner may proceed directly to the carbonator or the regenerated sorbent may be directed to a hydrator before entering the carbonator. A purge and make-up stream may also be used in the inventive process.
The introduction of carbonaceous material into the carbonator also provides advantages over the prior art. Specifically, the excess carbonaceous material added to the carbonator can be used as fuel in the calciner. This reduces the coal requirement due to the heat produced by the combustion of the excess carbonaceous material. In addition, the exothermicity of carbonaceous material combustion also enables lower inlet flue gas temperatures resulting in greater heat recovery in the steam turbine cycle prior to the carbonator. Accordingly, not only are embodiments of the inventive concept removing CO2, SOx and NOx simultaneously, but the carbonaceous material is helping to increase the efficiency of the calciner by providing fuel.